1. Field of the Invention
This invention relates to a method of making a body cavity insert, which method comprises forming an impression of a body cavity (e.g. an ear canal), using the impression to form a mould cavity in a mould, and introducing moulding material into the mould cavity to form the body cavity insert. Although the invention may be applied to making gum and mouth shields for sportsmen or for other dental applications, e.g. as an insert to straighten teeth, the invention is primarily concerned with making an ear insert.
2. Description of the Prior Art
Ear inserts for use in deaf aids or for other specialised applications, e.g. for aids in speech training, are currently usually made by a method which is time-consuming and of whose products only about 80% are normally satisfactorily useful for their intended purpose.
This known method involves forming an impression of an ear canal and dipping the impression into molten wax to form a wax coating on the surface of the impression. The purpose of the wax coating is to compensate for removal of material from a moulded insert by polishing which, as mentioned later, it is necessary to carry out after the moulding operation. The wax-coated impression is inserted into a body of soft plaster of Paris contained in the lower half of a two-part flask, the lower half of the flask and plaster when set forming the lower half of a two-part plaster mould. After the plaster has set its upper surface is coated with a separator solution containing a material for facilitating separation of the two mould parts when they are subsequently placed together in contact. When the separator solution has dried, the upper half of the flask is filled with soft plaster of Paris and the two flask parts brought and held together. After the plaster in the upper flask part has set hard, the two flask parts are separated and the impression is removed. An acrylic moulding material is then introduced into the resulting mould cavity after wax has been removed and the cavity treated with separator solution and allowed to dry. The two parts of the flask are then brought and secured together and the flask is pressurised for ten to fifteen minutes and the acrylic material cured for from two to three hours whilst the flask is retained in a clamp. The moulded insert is then removed, trimmed and polished, considerable polishing being required due to the rough surface of the insert occasioned by the comparatively rough finish of the plaster mould cavity.
Although removal of material by the polishing is compensated for by the wax coating on the impression initially used, the wax coating is in practice never evenly distributed over the surface of the impression. This variation in the thickness of the wax coating, together with inevitable variations in the degree of polishing as dictated by surface roughness variations between one moulded insert and another, mean that in a certain proportion of products the insert does not form a snug fit when placed in a human ear canal. If the insert does not form a snug fit in the ear canal, acoustic feedback may occur in use, making the insert unsatisfactory.